Method for determining an optimal absorber stack geometry of a lithographic reflection mask

ABSTRACT

The present invention relates to a method for determining an optimal absorber stack geometry of a lithographic reflection mask comprising a reflection layer and a patterned absorber stack provided on the reflection layer, the absorber stack having a buffer layer and an absorber layer. The method is based on simulating aerial images for different absorber stack geometries in order to determine process windows corresponding to the absorber stack geometries. The optimal absorber stack geometry is identified by the maximum process window size. The invention further relates to a method for fabricating a lithographic reflection mask and to a lithographic reflection mask.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for determining an optimalabsorber stack geometry of a lithographic reflection mask and to amethod for fabricating a lithographic reflection mask and to alithographic reflection mask.

BACKGROUND OF THE INVENTION

The fabrication of highly integrated electrical circuits with smallstructural dimensions requires special structuring procedures. One ofthe most common procedures is the so-called lithographic structuringmethod. This method comprises applying a thin layer of aradiation-sensitive photoresist on the surface of a semiconductorsubstrate disc, also referred to as wafer, and exposing the same byradiation transmitted through a lithographic mask. In the so-calledphotolithography, electromagnetic radiation is used. During the exposingstep, a lithographic structure located on the mask is imaged on thephotoresist layer by means of an exposure tool including the mask.Thereafter, the target structure is transferred into the photoresistlayer and subsequently into the surface of the wafer by performingdevelopment and etch processes.

One main demand of the semiconductor industry is the continuous powerenhancement provided by increasingly faster integrated circuits which isinterrelated to a miniaturization of the electronic structures. Thereby,the attainable resolution of the structures is generally limited by thewavelength of the applied radiation. In the course of this development,lithographic methods are performed with radiation having ever smallerwavelengths. At present, the smallest exposure wavelength used in thesemiconductor production is 193 nm which enables the fabrication ofminimal feature sizes of approximately 70 nm. For the nearer future, theapplication of the so-called 193 nm immersion lithography is intended,thus allowing minimal feature sizes of about 50 nm.

In order to achieve still smaller feature sizes, the so-called extremeultraviolet lithography (EUVL) is being developed which is based on theapplication of electromagnetic radiation in the extreme ultraviolet(EUV) region with a wavelength of 13.4 nm. According to plans of thesemiconductor industry, the EUV-lithography is considered to be used forthe fabrication of dense sub-40 nm and isolated sub-25 nm structures bythe end of this decade.

Since no refractive materials (lenses) exist for EUV radiation, theradiation has to be reflected by special mirrors, i.e. multilayerreflection elements, which are used in the corresponding exposuresystems for the exposure tools and for the lithographic masks. A typicalEUVL reflection mask comprises a carrier substrate, a multilayerreflection layer provided on the carrier substrate, and a patternedabsorber stack provided on the multilayer reflection layer which definesthe lithographic structure. The reflection layer usually consists of anumber of Si/Mo-bilayers, which are disposed upon each other. Thepatterned absorber stack typically comprises a buffer layer consistingfor example of SiO₂ and an absorber layer consisting for example of Cror TaN. The buffer layer of the absorber stack serves for protection ofthe multilayer reflection layer during the fabrication of the reflectionmask, in particular regarding repair processes of pattern defects.

In order to provide a high contrast of the so-called aerial image, i.e.the intensity distribution of the radiation imaged on a wafer, theabsorber stack of a conventional EUVL reflection mask in general has athickness which is relatively large compared to the wavelength of theradiation, and thus comprises a geometry with a high aspect ratio, i.e.a high ratio between the thickness and a lateral dimension. A highaspect ratio geometry of an absorber stack provokes some disadvantages,however. Since a lithographic reflection mask is irradiated in anoblique angle of approximately 60 relative to a vertical plane,shadowing effects generally occur. Due to an absorber stack with a highaspect ratio, these shadowing effects strongly affect the imagingquality of a lithographic process. In particular, structuredisplacements and alterations of lateral dimensions of the structures,also referred to as critical dimension (CD), occur.

Another consequence of a high aspect ratio geometry of an absorber stackis a reduction of the lithography process window, i.e. the range ofpossible defocus and intensity dose values corresponding to a tolerablerange of target critical dimension values. In general, focus andintensity dose settings of an exposure tool are not constant in alithographic process but comprise variations, e.g. due to a wafer'snon-flatness or due to fluctuations caused by the exposure tool. Due tosuch variations, a target critical dimension lies within a defined rage.A reduction of the process window increases the danger of rejections ofprocessed wafers having structures with intolerable critical dimensionvalues.

In order to solve these problems, different solution concepts have beenproposed. One concept is based on providing reflection masks havingstructured multilayer reflection layers instead of patterned absorberstacks in order to avoid shadowing effects, as e.g. described in DE 10123 768 A1. However, the application of such reflection masks havingpatterned reflection layers requires the additional development ofdefect inspection and repair methods. But in the case of dark defects,i.e. an area where too many reflective multilayers have been removed,repairing is practically impossible. Moreover, structured multilayerreflection layers have a lower stability in particular concerningcleaning procedures.

Other concepts include methods for determining improved absorber stacklayouts by performing optical or aerial image simulations in whichstructure elements of absorber stacks are laterally displaced in aniterative manner in order to compensate for or minimize thedisplacements of the target structures due to shadowing effects. Suchmethods, however, which are alike to optical proximity correctiontechniques (OPC) performed on standard transmittance masks, are verycomplex and time-consuming.

SUMMARY OF THE INVENTION

The present invention provides an improved method for determining anoptimal absorber stack geometry of a lithographic reflection mask.

The present invention also provides a lithographic reflection maskhaving an optimal absorber stack that overcomes or diminishes theabove-mentioned disadvantages of the prior art reflection masks havingabsorber stacks with a high aspect ratio.

The present invention also provides a method for fabricating alithographic reflection mask having an optimal absorber stack.

According to one embodiment of the present invention, there is a methodfor determining an optimal absorber stack geometry of a lithographicreflection mask comprising a reflection layer and a patterned absorberstack provided on the reflection layer is provided, wherein the absorberstack has a buffer layer and an absorber layer. In a first step, atarget pattern for a structure imaged on a substrate by means of anexposure tool including the lithographic reflection mask to reflect aradiation is defined. This target pattern comprises a critical dimensionrange of target critical dimension values.

Afterwards, an absorber stack geometry for the target pattern isdefined. This step includes defining a value of at least one absorberstack parameter, wherein the absorber stack parameter includes a bufferthickness, a buffer sidewall angle, an absorber thickness, an absorbersidewall angle and a lateral absorber stack dimension.

Subsequently, aerial images of the structure imaged on the substrate aresimulated for a predetermined range of defocus values. This range ofdefocus values represents e.g. typical focus failures caused by anexposure tool used in a lithographic process.

In a subsequent step, each aerial image is evaluated by applying apredetermined range of intensity threshold values to determinecorresponding critical dimension values of the structure imaged on thesubstrate. By applying a predetermined range of intensity thresholdvalues, e.g. typical intensity dose variations of an exposure tool orother process variations like developer concentration variations whichare equivalent to intensity dose variations are incorporated into thesimulation.

Afterwards, the obtained critical dimension values of the structureimaged on the substrate are compared to the critical dimension range oftarget critical dimension values of the target pattern to determine aprocess window. Thereby, the size of the process window is defined bydefocus and intensity threshold values corresponding to target criticaldimension values.

Subsequently, further absorber stack geometries for the target patternare defined by varying the value of the absorber stack parameter. Afterthat, the described steps of simulating aerial images, evaluating eachaerial image and comparing the obtained critical dimension values arerepeated for the further absorber stack geometries to determine furtherprocess windows.

Thereafter, the sizes of the determined process windows corresponding tothe defined absorber stack geometries are compared to determine theoptimal absorber stack geometry. Thereby, the optimal absorber stackgeometry is identified by the maximum process window size.

This method allows for determining an optimal absorber stack geometry ofa lithographic reflection mask in an accurate and reliable manner. Incontrast to the above-described OPC-alike simulation methods, theinventive method mainly focuses on varying only “vertical” parameters ofan absorber stack geometry such as the thicknesses and the sidewallangles of the absorber layer and of the buffer layer. As a consequence,the inventive method is more time-efficient.

Moreover, the inventive method deals with determining an optimalabsorber stack geometry in relation to a process window having a maximumsize. As a consequence, an absorber stack geometry of a lithographicreflection mask can be determined which provides stable lithographicprocess conditions despite unwanted and perturbing defocus settings andintensity dose variations.

The inventive method is particularly suited for determining an optimalabsorber stack having a reduced thickness compared to that of theabove-described conventional reflection masks having absorber stackswith a high aspect ratio geometry. Consequently, the inventive methodcontributes to determining an absorber stack geometry of a lithographicreflection mask having reduced shadowing effects. In this manner, theimaging quality of a lithographic process can be further enhanced.

In a preferred embodiment of the present invention, a range of values ofthe absorber stack parameter used in the simulation steps is defined onthe basis of intensity contrast values of simulated aerial images.

These aerial images are simulated in advance according to the simulationsteps described above and are analyzed with regard to the intensitycontrast i.e. the difference between the minimum and the maximumintensity values of an aerial image. Thereafter, values of the absorberstack parameter and thus absorber stack geometries corresponding tosufficiently high contrast values of these aerial images are predefined.Consequently, an optimal absorber stack geometry of a lithographicreflection mask can be determined which provides high contrasts ofaerial images in a lithographic process. In addition, the method fordetermining an optimal absorber stack geometry is sped up due to thepre-determination of values of the absorber stack parameter. Moreover,it is preferred to specifically predetermine low values of the bufferlayer and/or of the absorber layer thickness. In this manner, theresulting determined optimal absorber stack has a low thickness, so thatshadowing effects are reduced.

In another alternative embodiment of the present invention, a range ofvalues of the absorber stack parameter used in the simulation steps isdefined on the basis of intensity contrast values of simulatednear-fields of the radiation reflected on the lithographic reflectionmask.

In contrast to aerial images, optical near-fields only incorporate thereflection performance of the lithographic reflection mask and neglectthe optical properties of the exposure tool. As a consequence, thepredetermination of values of the absorber stack parameter can be spedup and can in particular be carried out independently from theproperties of an exposure tool. Thereby it is preferred to predefinevalues of the absorber stack parameter corresponding to sufficientlyhigh contrast values of the near-fields. In particular it isadditionally preferred to predefined low values of the buffer layerand/or of the absorber, layer thickness.

According to another embodiment of the present invention, there is amethod for fabricating a lithographic reflection mask comprising areflection layer and a patterned absorber stack provided on thereflection layer is provided, wherein the absorber stack has a bufferlayer and an absorber layer. In a first step, an optimal absorber stackgeometry of the lithographic reflection mask is determined by performingthe inventive method or one of the preferred embodiments of the methoddescribed above. Subsequently, a carrier substrate with a reflectionlayer and an absorber blank provided on the reflection layer isprovided. The absorber blank comprises an unpatterned buffer layer andan unpatterned absorber layer, wherein the thicknesses of the bufferlayer and of the absorber layer are defined according to the determinedoptimal absorber stack geometry. Thereafter, the absorber blank isstructured according to the target pattern to provide a patternedabsorber stack on the reflection layer, wherein sidewalls of the bufferlayer and of the absorber layer are provided with sidewall anglesaccording to the determined optimal absorber stack geometry.

This method allows for fabricating a lithographic reflection mask havingan optimal absorber stack, thus allowing an improved imaging quality ina lithographic process. The method is in particular suited for thefabrication of a lithographic reflection mask comprising an absorberstack with a reduced thickness. As described above, such a reflectionmask is characterized by reduced shadowing effects, therefore providinga further enhanced imaging quality in a lithographic process.

According to another embodiment of the present invention, a lithographicreflection mask is provided. This lithographic reflection mask comprisesa carrier substrate, a reflection layer provided on the carriersubstrate and a patterned absorber stack provided on the reflectionlayer. The absorber stack comprises a buffer layer and an absorberlayer, wherein sidewalls of the absorber layer have a sidewall angle ofat least ten degrees relative to a vertical plane. Such a lithographicreflection mask provides a good imaging quality in a lithographicprocess since shadowing effects are reduced. This particularly appliesto a small thickness of the absorber stack.

In a preferred embodiment of the present invention, sidewalls of theabsorber layer have a sidewall angle of at least twenty degrees relativeto a vertical plane in order to further reduce shadowing effects andtherefore enhance the imaging quality.

A comparable effect on the imaging quality is provided in yet anotherpreferred embodiment of the present invention, wherein sidewalls of thebuffer layer have a sidewall angle of more than zero degrees relative toa vertical plane.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below with reference to thedrawings and exemplary embodiments, in which:

FIG. 1 shows a perspective view of a conventional lithographicreflection mask being irradiated at an oblique angle and a schematicintensity profile.

FIG. 2 shows Bossung plots obtained from the simulation of aerial imagesof radiation reflected on a conventional lithographic reflection mask.

FIG. 3 shows a flow chart of a method according to the invention fordetermining an optimal absorber stack geometry of a lithographicreflection mask.

FIG. 4 shows a parameter space of simulated absorber stack geometrieswhich is used to determine an optimal absorber stack geometry.

FIG. 5 shows a schematic view of an optimal absorber stack determinedwith the aide of the parameter space of FIG. 4.

FIG. 6 shows another parameter space of a simulated absorber stackgeometries.

FIG. 7 shows a schematic view of an optimal absorber stack determinedwith the aide of the parameter space of FIG. 6.

FIG. 8 shows a flow-chart of a method according to the invention forfabricating a lithographic reflection mask.

FIG. 9 shows a schematic view of a lithographic reflection maskaccording to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic view of a section of a conventionallithographic reflection mask 1 which is particularly used in aEUVL-process. For this, the reflection mask is integrated into anexposure tool (not shown) and irradiated at an oblique angle of e.g. 6°by a EUVL-radiation 7 having a wavelength of e.g. 13.4 nm.

The reflection mask 1 comprises a carrier substrate 6 and a multilayerreflection layer 5 provided on the carrier substrate 6. The reflectionlayer 5 comprises a number of reflective bilayers, e.g. Si/Mo-layerswhich are capable of reflecting the incident radiation 7.

In order to define a target pattern for a structure which is imaged on asubstrate or wafer by means of the exposure tool including thelithographic reflection mask 1, the reflection mask 1 further comprisesa patterned absorber stack 2 provided on the multilayer reflection layer5. The absorber stack 2 comprises a patterned absorber layer 3 having anabsorber layer thickness A and a patterned buffer layer 4 having abuffer layer thickness B.

The patterned absorber layer 3, which consists e.g. of Cr or TaN, servesfor absorbing the radiation 7 in definite areas to define the structureimaged on a substrate. In order to provide a high absorption, theabsorber layer 3 is generally fabricated with a relatively largethickness. The absorber layer 3 of the reflection mask 1 has a thicknessA of e.g. 70 nm.

The buffer layer 4, which is located between the absorber layer 3 andthe reflection layer 5, serves for protection of the reflection layer 5during the fabrication process of the lithographic reflection mask 1 inparticular concerning repair processes of structural defects of theabsorber layer 3. The buffer layer 4 is generally structured like theabsorber layer 3 and has a thickness B of e.g. 60 nm.

FIG. 1 also depicts a lateral dimension L of the absorber stack 2 whichcorresponds to the width of a recess located between two structuralelements of the absorber stack 2. This lateral dimension L is consideredto be converted into a critical dimension of the structure imaged on asubstrate by means of the exposure tool including the lithographicreflection mask 1. The lateral dimension L can alternatively be definedas the width of a structural element of the absorber stack 2.

As can be seen from FIG. 1, the recess located between the structuralelements of the absorber stack 2 is provided with vertical sidewalls. Inother words, the sidewalls of the buffer layer 4 and of the absorberlayer 3 have sidewall angles of 0° relative to a vertical plane.

The absorber stack 2 of the depicted conventional reflection mask 1 ischaracterized by a relatively large thickness compared to the wavelengthof the radiation, which is given by the thicknesses A, B of the absorberlayer 3 and the buffer layer 4. Moreover, the absorber stack 2 ischaracterized by a high aspect ratio, i.e. a high ratio between thethickness of the absorber stack 2 and the lateral dimension L. The largethickness and the high aspect ratio of the patterned absorber stack 2 ofthe lithographic reflection mask 1 involves some disadvantages.

The high aspect ratio of the absorber stack 2 in combination with theirradiation of the reflection mask 1 at an oblique angle results inshadowing effects of the patterned absorber stack 2, thus affecting theintensity profile of the reflected radiation 7. This impact of shadowingeffects can be seen from the intensity profile 8 depicted in FIG. 1. Inrelation to the midpoint of the recess located between the structuralelements of the absorber stack 2, which is illustrated by thedash-dotted line in FIG. 1, the maximum intensity is displaced.Moreover, the intensity profile 8 has an asymmetrical shape. In alithographic process, such an intensity profile 8 provokes structurewidth alterations and structure displacements, thus decreasing theimaging quality.

Another consequence of the shadowing effects is a reduction of thelithography process window. The process window characterizes thepossible variations of defocus and intensity dose values whichcorrespond to a definite tolerable range of target critical dimensionvalues of a structure imaged on a substrate by means of an exposuretool. These variations result e.g. from a wafer's non-flatness orfluctuations caused by the exposure tool during the lithographicprocess.

A lithography process window and in particular its size can be evaluatedwith the aide of so-called Bossung plots. For this, FIG. 2 shows anumber of Bossung plots 9 which are obtained from a simulation of aerialimages. An aerial image is a normalized intensity distribution of aradiation imaged on a substrate by means of an exposure tool during alithographic process.

In general, a number of aerial images are simulated for a range ofdefocus values. By applying a range of intensity threshold values, whichcorrespond to the intensity dose variations of the exposure tool,critical dimensions of the structures imaged on a substrate can bederived. As a consequence, graphs representing the dependency betweencritical dimension and defocus values for constant intensity thresholdvalues, also referred to as Bossung plots 9, can be obtained.

The Bossung plots 9 allow for determining a process window 10, as shownin FIG. 2. This process window 10 defines a tolerable range of criticaldimension values and a range of defocus values. The correspondingtolerable intensity threshold values are consequently given by theBossung plots 9 running within these value ranges.

The above-described shadowing effects of the absorber stack 2 of theconventional reflection mask 1 result in a tilting or bending of theparabolic Bossung plots 9. The tilted shape of the Bossung plots 9 hasthe effect that a few Bossung plots 9 run through the defined criticaldimension and focus value ranges. In other words, the shadowing effectshave the impact of a reduction of the process window 10.

FIG. 3 shows a flow chart of a method according to the invention fordetermining an optimal absorber stack geometry of a lithographicreflection mask. Thereby, the lithographic reflection mask comprises areflection layer and a patterned absorber stack provided on thereflection layer, wherein the absorber stack has a buffer layer and anabsorber layer.

In a first step 41, a target pattern for a structure imaged on asubstrate or wafer by means of an exposure tool including saidlithographic reflection mask to reflect a radiation is defined. Thistarget pattern, which includes e.g. a line and space structure or acontact hole structure, comprises a critical dimension range of targetcritical dimension values.

In a subsequent step 42, an absorber stack geometry for said targetpattern is defined. This step 42 includes defining values of absorberstack parameters such as a buffer thickness, a buffer sidewall angle, anabsorber thickness, an absorber sidewall angle and a lateral absorberstack dimension.

After that, aerial images of the structure imaged on the substrate aresimulated for a range of predetermined defocus values (step 43). Thesimulation of aerial images is based on additional parameters such asoptical material parameters of the absorber layer and the buffer layerlike the absorption coefficient, and parameters characterizing theexposure tool such as the angle of incidence of the radiation, anumerical aperture, a partial coherence and a reduction factor.

In a subsequent step 44, each aerial image is evaluated by applying apredetermined range of intensity threshold values to determinecorresponding critical dimension values of said structure imaged on saidsubstrate. As mentioned above, these intensity threshold valuescorrespond to intensity dose values of the exposure tool. This step 44corresponds to an idealized resist system with infinite contrast.

Afterwards, the obtained critical dimension values of said structureimaged on the substrate are compared to the critical dimension range oftarget critical dimension values of said target pattern to determinedprocess window (step 45), wherein the size of the process window isdefined by defocus and intensity threshold values corresponding totarget critical dimension values. This step 45 can e.g. be performedwith the aide of the above-described Bossung plots.

Subsequently, further absorber stack geometries for the target patternare defined by varying the values of one or more of said absorber stackparameters (step 46) and the steps 43 to 45 are repeated for the furtherabsorber stack geometries to determine further process windows (step47).

At the end, the sizes of the determined process windows corresponding tothe defined absorber stack geometries are compared in a step 48 todetermine the optimal absorber stack geometry. Thereby, the optimalabsorber stack geometry is identified by the maximum process windowsize.

Concerning the absorber stack parameters, there are a number ofdifferent performing embodiments of the inventive method. One embodimentcomprises e.g. using definite constant values for the buffer layerparameters and varying the values of the absorber layer parameters.

For this, FIG. 4 shows a parameter space 11 of simulated absorber stackgeometries, which is used to determine an optimal absorber stackgeometry. The parameter space 11 includes information on the processwindow size for different values of the absorber sidewall angle(absorber slope) and different values of the absorber thickness. Thesize of the process window is indicated by a brightness scale, which canbe gathered from the bar located on the right-hand side. This barassigns different process window sizes, which are specified in variableunits, to different brightness graduations.

The process window sizes were determined by carrying out the inventivemethod depicted in FIG. 3. For this, the simulation of aerial images wasbased on a dense line and space structure as a target pattern having acritical dimension of 30 nm, an angle of incidence of the radiation of6° relative to a vertical plane, a numerical aperture NA=0.3, a partialcoherence Sigma=0.7 and a reduction factor of 5. For the buffer layerparameters, constant values, namely a thickness of 60 nm and a buffersidewall angle of 0° relative to a vertical plane were used.

In the parameter space 11 from FIG. 4, the drawn through circle marksthe absorber stack geometry of a conventional EUVL reflection mask,whereas the dotted circle marks the optimal absorber stack geometryproviding a maximum process window size. The optimal absorber stackgeometry is characterized by an absorber layer thickness of 30 nm and anabsorber side wall angle of 20° relative to a vertical plane.

A schematic view of an absorber stack 12 comprising an absorber layer 13and a buffer layer 14 having an optimal geometry according to theparameter space 11 from FIG. 4 is depicted in FIG. 5. In comparison to aconventional EUVL reflection mask, a reflection mask having the absorberstack 12 provides an enhanced imaging stability due to the largerprocess window. Moreover, the thickness of the absorber stack 12, whichis 90 nm, is reduced in comparison to the thickness of an absorber stackof a conventional reflection mask. The thickness of the absorber stack 2of the reflection mask 1 from FIG. 1 is e.g. 130 nm. Consequently,shadowing effects are reduced, thus improving the imaging quality.

FIG. 6 shows another parameter space 21 which was also compiled bycarrying out the inventive method from FIG. 3. Here, the simulation wasalso based on a line and space structure as target pattern having acritical dimension of 30 nm, an angle of incidence of the radiation of6° relative to a vertical plane, a numerical aperture NA=0.3, a partialcoherence Sigma=0.7 and a reduction factor of 5. Again, constant valuesfor the buffer layer parameters were used, namely a thickness of 10 nmand a buffer sidewall angle of 0° relative to a vertical plane.

The drawn through circle in the parameter space 21 again marks theabsorber stack geometry of a conventional EUVL reflection mask. Theoptimal absorber stack geometry marked by the dotted circle ischaracterized by a thickness of the absorber layer of 50 nm and anabsorber sidewall angle of about 25° relative to a vertical plane.

FIG. 7 shows a schematic view of an absorber stack 22 comprising anabsorber layer 23 and a buffer layer 24 having an optimal geometryaccording to the parameter space 21 from FIG. 6. A reflection maskhaving such an absorber stack 22 again provides a large process windowand an improved imaging quality due to a reduced thickness of the stack22, which is only 60 nm at hand.

These two simulation examples described with reference to FIGS. 4, 5, 6and 7 do not limit the scope of the inventive method, however. Regardingthe variations of the absorber stack parameters, alternative performingembodiments of the inventive method exist.

As an example, it is possible to use constant absorber layer and bufferlayer sidewall angles, e.g. 0° relative to a vertical plane, and to varythe values of the thicknesses of the absorber layer and of the bufferlayer to determine an optimal absorber stack geometry providing aprocess window with a maximum size. Additionally, the thus determinedvalues of the thicknesses can e.g. be kept constant and the values ofthe absorber layer and buffer layer sidewall angles are variedafterwards to determine a further optimised absorber stack geometryproviding a process window having a still larger size.

In a further alternative embodiment, the value of the buffer layerthickness is kept constant and the values of the absorber layerthickness and of the absorber layer and buffer layer sidewall angles arevaried. Moreover other alternative embodiments are imaginable.

In this context, it is also possible to use a lateral absorber stackdimension as a varying parameter. A value of this parameter can bedefined and e.g. be kept constant in the first instance to determine anoptimal absorber stack geometry providing a maximum process window.Afterwards, the value of the lateral absorber stack dimension is variedto determine a further optimised absorber stack geometry providing aprocess window having a still larger size. It is also possible to varythe value of the lateral absorber stack dimension at first and to keepit constant towards the end or to vary its value during, the wholesimulation performance.

Moreover, it is possible to define a range of values of the absorberstack parameters used in the simulation steps in advance on the basis ofintensity contrast values of simulated aerial images. The intensitycontrast of an aerial image is the difference between the maximum andthe minimum intensity values. In this context, it is preferred to definevalues of absorber stack parameters which provide a relatively highintensity contrast. With such a predefined range of absorber stackparameters used in the simulation steps, the inventive method depictedin FIG. 3 can be sped up. Particularly, it is additionally preferred topredetermine low values of the thicknesses of the buffer layer and/orthe absorber layer. In this manner, the resulting optimal absorber stackhas a low thickness in addition, thereby reducing shadowing effects.

Alternatively, it is possible to predefine a range of values of theabsorber stack parameters used in the simulation steps on the basis ofintensity contrast values of simulated near-fields. A near-field is theintensity distribution of the radiation reflected on the reflectionmask, thus neglecting the optical characteristics of an exposure tool.Consequently, the above-mentioned predetermination of values of absorberstack parameters can be sped up and can be carried out independentlyfrom the properties of an exposure tool. Accordingly, it is preferred topredefine values of absorber stack parameters providing relatively highintensity contrasts, and in particular low values of the buffer layerand absorber layer thicknesses.

Moreover further embodiments of the inventive method for determining anoptimal absorber stack geometry are imaginable, which representvariations or combinations of the above-described embodiments.

The inventive method depicted in FIG. 3 can also be expanded with aso-called resist model which accounts for the exposing and developingperformance of a photoresist layer provided on the irradiated substrateor wafer. With the aide of a resist model, an interaction of aerialimages with the photoresist layer and a subsequent developing process ofthe photoresist layer, and so the photoresist image or profile of thetarget pattern on the surface of the substrate or wafer can besimulated. For such an expanded simulation method, additional parametersaccounting for photoresist properties are included in the simulation. Inaddition, the size of the process window has to be defined in anothermanner, e.g. defocus and intensity dose values corresponding to a rangeof critical dimensions of the photoresist profile.

FIG. 8 shows a flow chart of a method according to the invention forfabricating a lithographic reflection mask comprising a reflection layerand a patterned absorber stack provided on the reflection layer, whereinthe absorber stack has a buffer layer and an absorber layer. In a firststep 51, an optimal absorber stack geometry of the reflection mask isdetermined by performing one of the above-described embodiments of thesimulation method.

In a subsequent step 52, a carrier substrate with a reflection layer andan absorber blank provided on the reflection layer is provided. Theabsorber blank has an unpatterned buffer layer and an unpatternedabsorber layer, wherein the thicknesses of the buffer layer and of theabsorber layer are defined according to the determined optimal absorberstack geometry.

In a further step 53, the absorber blank is structured according to thetarget pattern used in the simulation method to provide a patternedabsorber stack on the reflection layer. Thereby, sidewalls of the bufferlayer and of the absorber layer are provided with sidewall anglesaccording to the determined optimal absorber stack geometry. Ifnecessary, also a lateral absorber stack dimension is provided accordingto the determined optimal stack geometry.

This method makes it possible to fabricate a lithographic reflectionmask having an optimal absorber stack, which provides an enhancedimaging quality in a lithographic process. The fabrication methodparticularly allows for fabricating a reflection mask having an absorberstack with a reduced thickness and consequently reduced shadowingeffects. Regarding the reduced thickness of the absorber stack, thestructuring step 53 is also facilitated. Moreover, the method is suitedfor fabricating reflection masks having optimal absorber stacks, whereinthe buffer and absorber layers particularly feature sidewalls havingsidewall angles of more than zero degrees relative to a vertical plane.

For this, FIG. 9 shows a schematic view of a lithographic (EUVL)reflection mask 31 according to a preferred embodiment of the presentinvention. This reflection mask 31 can favorably be fabricated accordingto the fabrication method depicted in FIG. 8. The reflection mask 31comprises a carrier substrate 6 and a multilayer reflection layer 5provided on said carrier substrate 6. The reflection mask 31 furthercomprises an absorber stack 32 provided on the reflection layer 5. Theabsorber stack 32 comprises a buffer layer 34 and an absorber layer 33,and preferably has a thickness which is reduced in comparison to thethickness of an absorber stack of a conventional EUVL reflection mask.

The absorber layer 33 features sidewalls having a sidewall angle of atleast 20° relative to a vertical plane. The buffer layer featuressidewalls having a sidewall angle of more than 0° relative to a verticalplane. As a consequence, shadowing effects are widely reduced whenexposing the reflection mask 31 to a radiation. In order to furtherreduce shadowing effects, the absorber layer 33 preferably featuressidewalls having a sidewall angle of at least 30° to a vertical plane.

In contrast to the reflection mask 31 shown in FIG. 9, reflection maskswith absorber stacks having alternative geometries are imaginable. It isfor example possible to provide an absorber stack wherein the absorberlayer and the buffer layer comprise sidewalls having equal sidewallangles of preferably more than 0° to a vertical plane.

1. A method for determining an optimal absorber stack geometry of alithographic reflection mask comprising a reflection layer and apatterned absorber stack provided on the reflection layer, the absorberstack having a buffer layer and an absorber layer, comprising: a)defining a target pattern for a structure imaged on a substrate by anexposure tool including the lithographic reflection mask to reflect aradiation, the target pattern comprising a critical dimension range oftarget critical dimension values; b) defining an absorber stack geometryfor the target pattern including defining a value of at least oneabsorber stack parameter, the absorber stack parameter includes a bufferthickness, a buffer sidewall angle, an absorber thickness, an absorbersidewall angle and a lateral absorber stack dimension; c) simulatingaerial images of the structure imaged on the substrate for apredetermined range of defocus values; d) evaluating each aerial imageby applying a predetermined range of intensity threshold values todetermine corresponding critical dimension values of the structureimaged on the substrate; e) comparing the obtained critical dimensionvalues of the structure imaged on the substrate with the criticaldimension range of target critical dimension values of the targetpattern to determine a process window, wherein the size of the processwindow is defined by defocus and intensity threshold valuescorresponding to target critical dimension values; f) defining furtherabsorber stack geometries for the target pattern by varying the value ofthe absorber stack parameter; g) repeating steps c) to e) for thefurther absorber stack geometries to determine further process windows;and h) comparing the sizes of the determined process windowscorresponding to the defined absorber stack geometries to determine theoptimal absorber stack geometry, wherein the optimal absorber stackgeometry is identified by the maximum process window size.
 2. The methodaccording to claim 1, wherein a range of values of the absorber stackparameter used in the simulation steps is defined on the basis ofintensity contrast values of simulated aerial images.
 3. The methodaccording to claim 1, wherein a range of values of the absorber stackparameter used in the simulation steps is defined on the basis ofintensity contrast values of simulated near-fields of the radiationreflected on the lithographic reflection mask.
 4. A method forfabricating a lithographic reflection mask comprising a reflection layerand a patterned absorber stack provided on the reflection layer, theabsorber stack having a buffer layer and an absorber layer, comprising:determining an optimal absorber stack geometry of the lithographicreflection; providing a carrier substrate with a reflection layer and anabsorber blank provided on the reflection layer, the absorber blankhaving an unpatterned buffer layer and an unpatterned absorber layer,wherein the thicknesses of the buffer layer and of the absorber layerare defined according to the determined optimal absorber stack geometry;and structuring the absorber blank according to the target pattern toprovide a patterned absorber stack on the reflection layer, whereinsidewalls of the buffer layer and of the absorber layer are providedwith sidewall angles according to the determined optimal absorber stackgeometry.
 5. A lithographic reflection mask, comprising: a carriersubstrate; a reflection layer provided on the carrier substrate; and apatterned absorber stack provided on the reflection layer comprising abuffer layer and an absorber layer, wherein sidewalls of the absorberlayer have a sidewall angle of at least ten degrees relative to avertical plane.
 6. The lithographic reflection mask according to claim5, wherein sidewalls of the absorber layer have a sidewall angle of atleast twenty degrees relative to a vertical plane.
 7. The lithographicreflection mask according to claim 5, wherein sidewalls of the bufferlayer have a sidewall angle of more than zero degrees relative to avertical plane.